The first rocks on Earth were igneous rocks. Igneous rocks are formed by the cooling and hardening of molten material called magma . The word igneous comes from the Latin word ignis, meaning fire. There are two types of igneous rocks: intrusive and extrusive. Intrusive igneous rocks form within Earth's crust ; the molten material rises, filling any available crevices, into the crust, and eventually hardens. These rocks are not visible until the earth above them has eroded away. Intrusive rocks are also called plutonic rocks, named after the Greek god Pluto, god of the underworld. A good example of intrusive igneous rock is granite . Extrusive igneous rocks form when the magma or molten rock pours out onto the earth's surface or erupts at the earth's surface from a volcano . Extrusive rocks are also called volcanic rocks. Basalt , formed from hardened lava , is the most common extrusive rock. Obsidian , a black glassy rock, is also an extrusive rock.
Igneous rocks are classified according to their texture and mineral or chemical content. The texture of the rock is determined by the rate of cooling. The slower the cooling, the larger the crystal. Intrusive rock can take one million years or more to cool. Fast cooling results in smaller, often microscopic, grains. Some extrusive rocks solidify in the air, before they hit the ground. Sometimes the rock mass starts to cool slowly, forming larger crystals , and then finishes cooling rapidly, resulting in rocks that have crystals surrounded by a fine, grainy rock mass. This is known as a porphyritic texture.
Most of Earth's minerals are made up of a combination of up to ten elements. Over 99% of Earth's crust consists of only eight elements (oxygen , silicon , aluminum , iron , calcium, sodium, potassium, and magnesium). Most igneous rocks contain two or more minerals, which is why some rocks have more than one color. For example, the most common minerals in granite are quartz (white or gray), feldspar (white or pinks of varying shades), and mica (black). The amount of a specific element in a mineral can determine a color or intensity of color. Because of the way granite is formed, the different composition of minerals is easy to see. It is difficult to see the distinct composition of some extrusive rocks, like obsidian, due to their extremely fine texture. Igneous rocks contain mostly silicate minerals and are sometimes classified according to their silica content. Silica (SiO2) is a white or colorless mineral compound. Rocks containing a high amount of silica, usually more than 50%, are considered acidic (sometimes the term felsic is used), and those with a low amount of silica are considered basic (or mafic ). Acidic rocks are light in color and basic rocks are dark in color.
Essentially, Earth's continents are slabs of granite sitting on top of molten rock. The crustal plates of Earth are continually shifting, being torn open by faults, and altered by earthquakes and volcanoes. New igneous material is continually added to the crust, while old crust falls back into the earth, sometimes deep enough to be remelted. Igneous rocks are the source of many important minerals, metals , and building materials.
